This invention relates to an apparatus for use with electronic article surveillance (EAS) systems, and in particular, to an apparatus for deactivating EAS markers used in such systems.
In the field of electronic article surveillance, EAS markers which incorporate some type of magnetic sensor assembly are placed on articles of merchandise to prevent unauthorized removal of the merchandise from a store. In a widely used type of EAS marker, the magnetic sensor assembly includes a magnetomechanical active element and a biasing or control element that is formed of a hard or semi-hard magnetic material. An EAS system using such a magnetomechanical marker is disclosed in U.S. Pat. No. 4,510,489, issued to Anderson et al.
To place the marker in an active condition, the control element is magnetized so that it provides a biasing field for the active element. The biasing field is such that the active element mechanically vibrates in response to an interrogation signal, thereby generating a detectable signal at the frequency of the interrogation signal. Deactivation of the marker is accomplished by changing the magnetic state of the control element in such a manner that the active element is no longer biased to mechanically vibrate in response to the interrogation signal.
Known types of devices for deactivating magnetomechanical EAS markers include active and passive devices. Active devices include coils, power supplies, and active electronic circuits which cause the coils to be driven with a time-varying current so as to produce a time-varying electromagnetic field. Exposing the magnetomechanical marker to this varying field causes the control element to be degaussed. As a result, the control element no longer provides a bias field for the active element, and the active element does not mechanically vibrate upon exposure to the interrogation signal.
On the other hand, passive deactivation devices do not include an active electronic element for generating an electromagnetic field. An example of a passive deactivation device will now be described with reference to FIG. 1, in which reference numeral 10 generally indicates a deactivation pad. The deactivation pad 10 includes thin strips 12-1, 12-2, 12-3, 12-4, etc. of permanently magnetized material. The strips 12 are arranged in parallel to each other extending in the longer dimension of the pad 10, and with alternating magnetic polarities. For example, the strips 12-1, 12-3, etc. (indicated by shading in FIG. 1) are arranged with a north pole upwards while the strips 12-2, 12-4, etc., interleaved with the other strips, have a south pole oriented upwardly. Typical dimensions for the pad 10 are about 4 inches by 6 inches, with each of the strips being about 0.25 inch wide so that a pole density of 4 poles per inch is provided in the direction of the width of the pad. The strips 12 are typically packaged between a keeper plate (not shown) provided underneath the strips 12 and a cover plate (also not shown) on the top of the strips 12. The overall height of the pad 10 is about 0.25 inch.
A deactivation pad 10 of the type just described is substantially less expensive to manufacture than active devices because of the absence of electronic circuitry, power supplies and so forth. However, for reasons to be described below, the deactivation pad 10 does not always function reliably to deactivate markers.
A typical marker 13 in an active state is schematically illustrated in FIG. 2. The marker 13 includes a parallelogram-shaped element of bias material 14, which functions as a control element, and is mounted in a housing 16 (indicated in phantom) with a magnetostrictive active element (not shown). The control element 14 may be formed of an alloy such as Arnokrome-3, Crovac 10/130 or Vicalloy, or of an amorphous material such as Metglas 2605 TCA (transformer core alloy--Fe.sub.78 Si.sub.9 B.sub.13) annealed according to a process disclosed in U.S. Pat. No. 5,351,033 so that the amorphous material is crystallized and exhibits semi-hard magnetic properties. To place the marker in an active condition, the control element 14 is magnetized along its length to form magnetic pole regions 18 and 20. For example, the pole region 18 may be a north magnetic pole in which case the pole region 20 is a south magnetic pole. A typical length for the control element 14 is about 11/2 inches.
The marker 13 is deactivated by being brought into contact with, or swept across, the top surface of the pad 10. If this is done with the marker oriented as indicated by the double arrow 22 (FIG. 1), i.e. with the marker extending transversely across several of the strips 12, then the alternating polarity of the strips causes the pattern of magnetization of the control element 14 to be broken up along the length of the control element. In this condition, the control element 14 does not provide a bias field that will result in a substantial magnetomechanical response by the active element of the marker. However, if the marker, when placed in contact with the pad 10, is oriented as indicated by the double arrow 24, i.e. with its length extending in the same direction as the strips 12, then the contact with the pad 10 results in a condition like that illustrated in FIG. 3. As shown in FIG. 3, the pattern of magnetization of the control element 14 resulting from contact with the pad 10 includes pole regions 26, 28 and 30 which extend along substantially the entire length of the control element 14. With the pattern of magnetization shown in FIG. 3, the control element 14 typically continues to exhibit a degree of remanent magnetization along its length and may be capable of providing a sufficient bias field along its length to cause the active element to exhibit a magnetomechanical response to an interrogation signal. As a consequence, the marker remains active and may generate a false alarm.
It is, therefore, an object of the present invention to provide an improved apparatus for deactivating magnetomechanical EAS markers.
It is a further object of the present invention to provide a marker deactivating apparatus that is both reliable and relatively inexpensive.